First working version of install for the packaging build.

Changes made to Gordon's code:

1.  I've added a top-level Makefile which does some of
    what the build script does, just building and
    installing the two libraries and the gpio command
    at present.

2.  In the library Makefiles this line assumes that the
    binary has been written into DESTDIR, without PREFIX:
      @ln -sf $(DESTDIR)$(PREFIX)/lib/libwiringPiDev.so.$(VERSION) $(DESTDIR)/lib/libwiringPiDev.so
    This doesn't seem to be the case, however... so I've added
    a PREFIX. Probably I've missed something about how make
    should be invoked, or...?

3.  The package build process requires that DESTDIR is
    used for everything -- so calling "ldconfig" without
    parameterising it for DESTDIR causes an error. I've
    changed it to use DESTDIR (and -n) for now; there is
    probably a better way using "postinst" rules.

4.  The ./devLib/Makefile does a -I., but this doesn't
    pick up the headers in DESTDIR, so I've changed INCLUDE
    there to do so (in the same way the gpio/Makefile does).

5.  The gpio build needs the core library to be build
    first, so I've put both the bare "make" and the "make
    install" under the top-level "install" target.
    Probably non-optimal.

6.  I have changed the gpio install to use the "install"
    command, which doesn't assume that the process is
    running as root.

7.  I have added a compressed version of the man page,
    as that seems to be expected.